The invention herein relates to the duplicating of strip film and specifically is directed to a method and apparatus for duplicating positive film of the silver halide type on a much more economical type of electrophotographic film.
The duplicating of strip film is principally practiced in the cinema field where multiple copies of a master are made. The basic technique in such duplication of using a flow type camera is also utilized in the invention. All film for taking systems in use today is silver halide film which is expensive and which, in cases where large quantities of film are retained, ties up great stores of silver, a scarce metal. For example, aside from use in the cinema field, there are vast accumulations of exposed silver halide film held in information storage and retrieval systems.
Xerographic and electrofacsimile apparatus and techniques are well-known. In these processes, a photoconductive coating on an ohmic surface is charged by corona or the like, is exposed to the image of a scene and is toned so that the latent charge image becomes visible. In the electrofacsimile (electrofax) process, the toned image is fixed directly onto the photoconductive coating. The most familiar one of these is the zinc oxide-resin coating on a sheet of conductive paper. This produces a copy of the image on an opaque sheet. The electrophotographic members of known electrostatic processes are notoriously slow. It has never been practical, so far as known, to make copies from photographic silver halide film onto these prior art members because of their low photographic speed. Additionally, the resolution and grey scale of prior art electrostatic copies are so inferior to those of conventional photographic film that it is impractical if not impossible to reproduce comparable photographic images on xerographic or electrofax members.
The advent of the electrophotographic film of the copending application makes reproduction of images from conventional photographic film onto electrostatic members feasible and highly economical, the duplication thus enabling the salvage of the silver halide film and the recovery of the silver therefrom. Such electrophotographic film is a highly flexible, high speed, high resolution material with a hard and abrasion resistant surface. Its other attributes are fully detailed in the said copending application.
In the duplication of film in the cinema art, where a copy of a master is being made, the technique of image enhancement is practiced. The master is carefully edited and the editor prepares a program of image enhancement which involves generally the variation in the amount of light used to project the image from the master onto the copy film. In other words, the editor chooses the proper exposure for the film as it is being duplicated to provide the optimum film density for viewing. According to the invention this is accomplished automatically. The advantages are manifold since the speed of the flow type camera need not be decreased and there is no need for pre-editing the film.
The method and structure for automatic image enhancement operate on the charging means for the electrophotographic film, which in effect involves varying the tonability of the film. There is no parallel or equivalent of this in conventional photographic film whose sensitivity is fixed when the film is manufactured and whose image density can only be controlled by changing the exposure of an image or the amount of incident light or both.